The invention relates to the field of electronics, and, more particularly, to a system and method for adjusting a radio-frequency emission spectrum.
In a mobile telephone, the useful signal emitted, in this instance voice, is a low-frequency signal transposed into a high-frequency signal using an IQ mixer with the aid of a high-frequency signal originating from a local oscillator. The output spectrum of the signal provided by the mixer therefore contains a so-called useful line. This corresponds to the useful signal transposed to high frequency, and to the spurious lines. The spurious lines in this instance are the line of the local oscillator signal as well as the image line of the useful signal.
When a mobile telephone is locked for a predetermined duration, e.g., 12.5 ms, onto a transmission frequency channel, it is necessary for the spectrum of the signal emitted to occupy the least possible pass band to disturb as little as possible the other transmission frequency channels which may be allotted to other radio-frequency transmitters/receivers. Also, the local oscillator line and the image line can disturb the other channels. The adjusting of the mixer is therefore aimed at canceling, or at the very least at decreasing the levels of the spurious lines, i.e., the level of the image line and of the local oscillator line.
Adjustment includes tailoring the symmetry of the low-frequency input signals of the mixer to compensate for the internal asymmetries of this component which are responsible for increasing the image and local oscillator lines of the output spectrum. In this regard, two parameter test signals are delivered respectively to the input of the two paths I and Q of the mixer. Each test signal typically has a DC component and a periodic component. The periodic component is typically sinusoidal. The periodic components of the two test signals are mutually phase-shifted. The adjustment includes tailoring the value of the DC components for a fixed value of the frequency of the local oscillator signal. The adjustment also includes adjusting the ratio of the amplitudes of the two sine components and the value of the phase shift to minimize the levels of the spurious lines.
A conventional method for carrying out this tailoring includes performing a sweep on the input path I, and then on the path Q, for each possible local oscillator frequency associated with a transmission channel. The various values are measured for the parameters of the test signals, along with the levels obtained for the image and local oscillator lines. This operation is repeated while refining the sweep step size until the desired rejection is obtained. Parameter values are thus obtained making it possible to obtain very low levels for the image and local oscillator lines.
However, this method has the drawback of requiring a very considerable number of measurements, typically on the order of a few tens to a few hundreds for a local oscillator frequency value. Consequently, this provides a very long adjustment time. Moreover, the adjustment is currently performed with a nominal supply voltage. Furthermore, a check is made as to whether, at the end of the life of the battery of the telephone, the quality of the output spectrum of the mixer is still in accordance with the desired specifications.
However, during the operation of a telephone, the variations in the supply voltage and/or in the temperature in particular, may modify certain characteristics of the transistors forming certain elements of the mixer. This may consequently modify its behavior, and in particular, the internal asymmetries. Hence, this results in a misfit between the adjustment parameters established during the test with the nominal voltage. Optimal adjustment of the mixer would then require a new adjustment of the parameters of the test signals to be performed in real time. Such a real-time adjustment, i.e., for a duration in which the telephone is locked onto a transmission channel, is totally incompatible with the duration of adjustment currently required.
Furthermore, the effectiveness of the current method is poor as it becomes closer to the solution. This is because the measurement noise, although very low, tends to trap the iterative search algorithm in a local minimum. This leads to obtaining reference values or optimal values for these parameters which are different from the theoretical reference values.
An object of the invention is to provide an adjustment for the output spectrum of an IQ mixer that requires a very low number of measurements to determine the parameters of the test signals applied to the IQ mixer.
Another object of the invention is provide an adjustment to the input signals applied to the IQ mixer which can be implemented in real time in a mobile telephone.
A further object of the invention is to increase the effectiveness of the above described adjustment.
The invention results, in particular, from the fact that it has been found that it is possible to correlate the power level of the image and local oscillator lines of the output spectrum with the various parameters of the test signals via two parabolic mathematical relations. On the basis of this, the determination of the reference or optimal values for the relevant parameters, and reference values corresponding to theoretically zero levels of these spurious lines can be performed by a numerical calculation on a reduced number of measurements. This is typically a few measurement points.
The invention is therefore distinguished from the prior art, in particular, by the fact that the determination of these reference values is performed by a numerical calculation, while in the conventional method this determination is performed solely by observing the measured levels of a very large number of points. This is done for selecting the values of the parameters that have led to a minimum level.
The invention thus makes it possible to decrease the duration of the adjustment in a ratio of greater than ten. This decreases the cost of products incorporating such a mixer given that the cost of the test represents a considerable percentage of the cost of these products. Furthermore, the duration of such an adjustment according to the invention makes it possible, if necessary, to implement such an adjustment in real time during the operation of a mobile telephone.
Stated otherwise, the invention therefore provides a process for adjusting the level of the spurious lines of the output frequency spectrum of a single-sideband frequency mixer. This process comprises the steps of:
delivering at the input of the two paths of the frequency mixer two mutually phase-shifted test signals each having a plurality of parameters;
measuring the level of each of the spurious lines for different test values of the various parameters of the two test signals; and
determining reference values for the various parameters making it possible to minimize the levels of the spurious lines.
According to a general characteristic of the invention, the reference values are determined by a numerical calculation performed on a predetermined number of different test values of the parameters and corresponding measured values of the levels. This is done with respect to two parabolic type relations linking the levels of the two spurious lines with the parameters to minimize the duration of adjustment by using a reduced number of test values.
According to one mode of implementation of the process, each test signal comprises a DC component and a periodic component, e.g., a sine component. The DC components of the two test signals form respectively first and second parameters. The ratio of the amplitudes and the relative phase-shift of the two periodic components of the two test signals form respectively third and fourth parameters. The spurious lines comprise an image line and a local oscillator line. The first and second parameters are linked to the level of the local oscillator line by a first parabolic relation, while the third and fourth parameters are linked to the level of the image line by a second parabolic relation. The reference values of the first and second parameters are thus determined by only making measurements of the level of the local oscillator line, while the reference values of the third and fourth parameters are determined by only making measurements of the level of the image line.
In a general manner, several variations are possible for determining these reference values. This is done with regard to the correlation of the paraboloid type relationships linking the levels of the spurious lines with the values of the parameters of the test signals. A first variation of the invention includes calculating the position of the optimum of the relevant parabolic relation by determining the point at which the derivative is zero.
More precisely, according to this variation the reference value of each parameter is determined by using at least four different test values of the parameter to obtain at least four corresponding measured values of the level. On the basis of the test values and the corresponding values of level, the value of the parameter corresponding to a zero derivative of the corresponding parabolic relation is then calculated. This value forms the reference value for the relevant parameter.
In practice, the number of test values used for this variation is equal to four. On the basis of the four test values and the four measured values of level, it is possible to form two triplets of consecutive test values. For example, the first, the second and the third are one triplet, and the second, the third and the fourth are a second triplet. Advantageously, the difference is then calculated between the two measured values of level corresponding to the extreme test values of each triplet. For example, the difference between the levels corresponding to the first and third test values, and the difference between the levels corresponding to the fourth and second test values.
The ratio is then taken between this difference and the difference of the two extreme values of the relevant triplet. In this instance, the difference is between the third and first test values and, the difference is between the fourth and second test values. Thus, for each triplet, a derivative value associated with the median test value of the triplet is obtained. In this instance, a derivative value is associated with the second test value and a derivative value is associated with the third test value. The reference value is next calculated on the basis of the equation for the straight line passing through two derivative values associated with two different median test values. In this instance, the equation for the straight line passes through the two previously calculated derivative values.
Other variations of the invention include taking a number of measurement points and solving a linear system of several equations having several unknowns. This may be performed according to the known Kramer method. More precisely, according to a second variation of the invention, the reference value of each parameter is determined by using only three different test values of the parameter to obtain three corresponding measured values of level. Also obtained is a linear system of three equations having three unknowns on the basis of the corresponding parabolic relation of the three different test values and of the three corresponding measured values of the level. The linear system is then solved.
Another variation of the invention is even more advantageous since it requires an even smaller number of measurement points. This variation makes it possible to determine the respective reference values of the first and second parameters by using only four different pairs of test values for the first and second parameters. This is done to obtain four corresponding measured values of level and to obtain a first linear system of four equations having four unknowns. This is done on the basis of the corresponding parabolic relation of the four test value pairs and of the four corresponding measured values of level. This first linear system is then solved.
In a similar manner, the respective reference values of the third and fourth parameters are determined by using only four different pairs of test values for the third and fourth parameters. This is done to obtain four corresponding measured values of the level and to obtain a second linear system of four equations having four unknowns. This is done on the basis of the corresponding parabolic relation of the four test value pairs and of the four measured values of level. This second linear system is then solved.
The subject of the invention is also related to a system for adjusting the level of the spurious lines of the output frequency spectrum of a single-sideband frequency mixer. The system comprises a delivery circuit for delivering at the input of the two paths of the frequency mixer two mutually phase-shifted test signals each having a plurality of parameters. The system further includes a measuring circuit for measuring the level of each of the spurious lines for different test values of the various parameters of the two test signals, and a determining circuit for determining reference values for the various parameters making it possible to minimize the levels of the spurious lines.
According to a general characteristic of the invention, the determination circuit for determining the reference values includes a processor able to perform a numerical calculation on a predetermined number of different test values of the parameters and corresponding measured values of levels. This is performed with regards to two parabolic relations linking the levels of the two spurious lines with the parameters.
Furthermore, the subject of the invention is also related to a mobile telephone comprising a single-sideband frequency mixer and a spurious signal spectral level adjusting system to allow implementation of the method according to the invention within the mobile telephone. Implementation of the method is performed in a production factory or real time during operation of the mobile telephone.